In a through hole, a redundant part which does not contribute to interlayer conductivity sometimes exists. In the case of transmitting a high speed signal via such a through hole in which the redundant part exists, the redundant part acts as an open stub, which causes resonance of the signal. Consequently, pass characteristics of a frequency attributed to a wavelength thereof deteriorate.
Then, the through hole is dug (excavated) with a drill, thereby removing the stub (see PTL 1).